GMSK modulation of signals is used in many digital communication systems such as GSM (Global System for Mobile Communication). GMSK modulation, which is a FSK (Frequency Shift Keying) modulation method using a Gaussian filter, is very efficient in using the available frequency spectrum. However, it suffers from intra-symbol interference (ISI), which causes degradation on the bits modulations. Therefore, digital communication systems using GMSK often apply equalizers, which are capable of compensating distortions due to ISI such as the Viterbi algorithm.
It is known in the art to use GMSK coupled with CDMA (Code Division Multiple Access). This technique is for example used in digital communication technology since GMSK offers very good spectrum efficiency. However, ISI caused by GMSK widens the correlation function of CDMA. This may cause inter-chip interference (ICI) due to the fact that the CCF (Cross Correlation Function) is usually no longer limited between ±1 chip. The larger the GMSK filter is, i.e. the lower the BT factor of the filter, the higher are the CCF values outside ±1 chip. The tracking performance for spread spectrum GMSK signals is therefore degraded compared to a BPSK (Binary Phase Shift Keying) modulated signal with the same chipping rate.
The European patent application EP1678837A1, published under the international publication number WO2005/043767A2, presents methods, apparatuses, and systems for transmitting and receiving spread spectrum GMSK signals. Transmission may involve obtaining a sequence of data symbols, obtaining a spread spectrum code comprising a sequence of spread spectrum chips, generating a sequence of pre-modulation chips by combining the sequence of data symbols with the spread spectrum chips, wherein for each data symbol, at least one of the pre-modulation chips is generated by taking into account at least the data symbol and at least one of the spread spectrum chips, performing GMSK modulation using the sequence of pre-modulation chips to produce a spread spectrum GMSK signal, and transmitting the spread spectrum GMSK signal.
A GNSS such as (NAVSTAR-)GPS (Global Positioning System) or the future European GNSS GALILEO use DSSS (Direct Sequence Spread Spectrum) modulation of the carrier frequencies reserved for the respective GNSS in order to transmit navigation data from the GNSS satellites of the space segment to GNSS receivers of the user segment. New signal designs for the modernization of GPS and for GALILEO introduce the use of longer spreading codes for improving the correlation properties and data less channels (pilot) in addition to data channels (data) for aiding weak signal tracking.